Modern digital communications typically require a means by which digital information is transmitted over an analog channel such as a telephone line frequency shift keying, or FSK, is one such method that has been commonly used in many communication protocols due to its simplicity and ease of implementation. In FSK, bits, or sequences of bits, are represented by signals of different frequencies. Non-coherent techniques for doing frequency shift keying detection are well known and can be found in any standard text on digital communications. The fundamental problem with these techniques is that they rely on complex filters and a heavy computational load to acquire the required estimates of the frequencies being transmitted. In applications where the power of the device and computational capacity need to be restricted, standard techniques of doing non-coherent detection do not work. An example would be non-coherent FSK detection in wireless communications, where the power requirements to operate a device are extremely restricted. Another example would be caller ID detection on a voice band modem, when the voice-band modem is operating in the low power mode. As such, there is a need for a technique which would allow one to do non-coherent FSK detection with minimal computational and power load.
To illustrate the principles associated with the invention, we shall select the problem of doing caller ID detection on a voice-band modem, when the modem is operating in the low power mode. Personal computer systems today are commonly equipped with a modem device that will allow data and fax communications via a standard telephone line. Current modem technology supports the ability to accept standard incoming calls and subsequently decode and display the caller ID information, if it is being transmitted by the telephone company. However, for this to work properly, the computer and hence the modem is required to be completely powered up (commonly referred to as being "awake") in order for the caller ID information to be captured and displayed. Most computers now have, or in the future will be built with, a power saving "sleep mode". While in this sleep mode, the computer modem will also be in a reduced energy consumption mode receiving only minimal power from a small auxiliary power supply. In most countries the caller ID data is transmitted by the telephone company only once at the start of the incoming call and in addition for some countries the recipient of the incoming call must declare an ability to receive the caller ID information before it can be transmitted by the telephone company. This usually leads to an extremely tight time schedule during which the modem must capture the caller ID information. If the modem is in the "sleep" mode, it usually takes more than two rings to switch from the "sleep" mode to the full power mode. This wake up time requirement does not allow the modem to respond to the telephone company in time, and as such the modem misses the caller ID information. For example, in United States, the caller ID information is transmitted only once by the telephone company between the first and second ring of the incoming telephone call, and the modem, if it is in the low power mode, will miss this information since the "wake up" time requirement is at least two rings for the modem.
Therefore, there exists a need to design a small additional circuit for the computer modem, capable of functioning on the very limited amount of power available from the auxiliary power supply; and with that limited amount of power, be capable of capturing the caller ID which can be processed and displayed by the modem when it "wakes up". The present invention provides an efficient, low-cost, effective solution to the above problem. These and other advantages of the present invention not specifically mentioned above will become clear within discussions of the present invention presented herein.